A number of fuel injection systems of engines utilize solenoids for controlling the operation of injector nozzles. This can be accomplished by directly operating the valve needle or indirectly by controlling, for example, the pressure level of injector nozzle servo oil by means of the solenoid valve. In both cases the operation is always based on the movement of the plunger of the solenoid. U.S. Pat. No. 6,240,901 by the applicant discloses a system based on indirect control.
An electric current flows through the coil of the solenoid when a source of electric energy is connected thereto. This produces a magnetic field that causes the plunger of the solenoid to move. In a solenoid, the electric current changes on the basis of the position of the plunger, and the current starts to decrease for a while especially when the plunger has started to move, thereby forming a local maximum value of the current. This information can be used for controlling the operation of the solenoid, especially it be used for observing the opening moment of the injectors nozzle. This information can also be used for making sure that the plunger has started to move.
Publication WO 03/007317 A1 discloses a method of controlling a solenoid. The starting point in this publication is, however, to define the moment when the injector nozzle has fully opened. The above-mentioned local maximum value of the current also occurs as the movement of the plunger stops. In the method described in the publication, the events during the movement of the injector nozzle are ignored.